Oil rigs are positioned at wellsites for performing a variety of oilfield operations, such as drilling a wellbore, performing downhole testing and producing located hydrocarbons. Downhole drilling tools are advanced into the earth from a surface rig to form a wellbore. Drilling muds are often pumped into the wellbore as the drilling tool advances into the earth. The drilling muds may be used, for example, to remove cuttings, to cool a drill bit at the end of the drilling tool and/or to provide a protective lining along a wall of the wellbore. During or after drilling, casing is typically cemented into place to line at least a portion of the wellbore. Once the wellbore is formed, production tools may be positioned about the wellbore to draw fluids to the surface.
During drilling, measurements are often taken to determine downhole conditions. In some cases, the drilling tool may be removed so that a wireline testing tool may be lowered into the wellbore to take additional measurements and/or to sample downhole fluids. Once the drilling operation is complete, production equipment may be lowered into the wellbore to assist in drawing the hydrocarbons from a subsurface reservoir to the surface.
The downhole measurements taken by the drilling, testing, production and/or other wellsite tools may be used to determine downhole conditions and/or to assist in locating subsurface reservoirs containing valuable hydrocarbons. Such wellsite tools may be used to measure downhole parameters, such as temperature, pressure, viscosity, resistivity, etc. Such measurements may be useful in directing the oilfield operations and/or for analyzing downhole conditions.
Attempts have been made to measure certain characteristics of a wellbore. Various techniques have been developed for measuring downhole parameters as described, for example, in US patent/application Nos. 20090204346, 20090153155, 20090072833, 20090090176, 20080288171, U.S. Pat. Nos. 7,689,363, 7,394,258, 7,397,250, 7,258,005, 5,457,396, 6,527,923, 7,066,282, 6,801,039, 5,677,631, 5,574,371, 5,345,179, 6,191,588, 3,879,644, 3,816,811, 4,608,983, 4,567,759, and 7,689,363. Techniques have also been developed for scanning as stated in publications “Formation Imaging with Microelectrical Scanning Arrays”, and “A General Framework for Constraint Minimization for the Inversion of Electromagnetic Measurements.”
More specifically, European Patent Application Nos. 102900084.2 and 10290083.4, filed by Applicant and incorporated herein by reference, relate to techniques for determining electrical parameters of downhole fluids.
In addition, International Patent Application No. PCT/EP2009/007637, filed by Applicant and incorporated herein by reference, relates to a tool and method for imaging a formation through a substantially non-conductive medium. The tool comprises a first circuitry for injecting a current into the formation, wherein a complex impedance to the current is measured. A second circuitry for determining a phase angle of an impedance of the nonconductive medium and a third circuitry for determining a component of the complex impedance that is orthogonal to the phase angle.
Despite the development of techniques for measuring downhole parameters, there remains a need to provide advanced techniques for determining parameters of downhole formations and/or wellbore fluids. It may be desirable to provide techniques that enhance downhole fluid and/or downhole formation measurements. It may be further desirable to provide techniques that correct for the effects of mud on downhole imaging and/or measurement. Preferably, such techniques involve one or more of the following, among others: accuracy of measurements, optimized measurement processes, operability in a variety of downhole fluids such as conductive and non-conductive muds, flexible measurement and/or analysis, operability in downhole conditions (e.g., at high temperatures and/or pressures), etc.